1. Field of the Invention
The present invention relates to a line transfer reading device with feedback.
2. Description of the Prior Art
It is well known in the prior art to read matrices of photosensitive detectors, such for example as diodes or MOS transistors, by the line transfer method.
In FIG. 1 has been shown schematically such a line transfer reading device. This device comprises a matrix 1 of photosensitive detectors d disposed in lines and columns. Conducting buses 2 connect the detectors disposed in the same column to the input of a line memory 3. A shift register 4 is provided for addressing a line of detectors of the matrix. The charges from the detectors of the adressed line are transfered to the line memory 3, then to a charge transfer shift register 5 whose output is connected to an amplifier 6.
To improve the efficiency in transferring the charges from bus 2, having high capacities of the order of a few picofarads for example, to the charge transfer shift register 5, a drive charge Q.sub.E is used. This charge is transferred from each stage of register 5 to the memory 3 then to a bus 2 and is then superimposed on the signal charges Q.sub.S which arrive on this bus, transferred to the line memory then to the register 5 to be read superimposed on the signal charges.
In FIG. 1, an embodiment has been shown in which a stage 7 ensures the injection of identical drive charges at the input of register 5. At the time when register 6 is read, a drive charge Q.sub.E is introduced into each stage of the register which has just been read.
The use of drive charges following such a path (register, memory, bus then bus, memory register and reading with the signal charges) is known under the name of "charge priming device or CPD. Thus the transfer efficiency is improved, that is to say reading of the charges coming from the buses.
Incomplete reading of the charges from the buses modifies the quiescent potential of the buses and introduces a mixture of informations from two lines of the matrix read successively. This results in a loss of vertical resolution when the device is used for image scanning, that is to say when two adjacent lines are read successively. The phenomenon observed is much more troublesome when the two lines read successively are not adjacent, that is to say when random addressing of the lines occurs, as for example in robotics. That results in the appearance of parasite signals.
Have been shown, in FIG. 2 on the left, the signal charge amounts Q.sub.S obtained at the output of register 5 when a line N is read. These signal charges are superimposed on a drive charge Q.sub.E, of constant value, shown with a broken line.
In FIG. 2, in the right hand part, has been shown the reading of line N+1 which does not contain any signal charge. We find, superimposed on the constant drive charge Q.sub.E, a residual signal charge Q.sub.R which has the same profile as the signal charge Q.sub.S of line N which is shown in FIG. 2 in the left hand part.